Speckle reduction in scanning display systems

ABSTRACT

Speckle effect in scanning display systems that employs polarized phase-coherent light is reduced by depolarizing the phase-coherent light using a depolarizer and scanning the depolarized light for producing desired images.

CROSS REFERENCE TO RELATED CASES

This US patent application claims priority from co-pending USprovisional patent application serial number 60/953,415 filed Aug. 1,2007, the subject matter of which is incorporated herein by reference inits entirety.

TECHNICAL FIELD OF THE DISCLOSURE

The technical field of this disclosure relates to the art of displaysystems; and more particularly to the art of display system employingphase-coherent light.

BACKGROUND OF THE DISCLOSURE

In recent years, lasers and other solid-state light sources capable ofproducing visible light have drawn significant attention as alternativelight sources to traditional light sources for use in imaging systemssuch as projection systems. This attention has been due to manyadvantages of these light sources, such as compact size, greaterdurability, longer operating life, higher efficiency, and lower powerconsumption.

Regardless of certain superior properties over traditional lightsources, solid-state light sources may produce unwanted artificialeffects, one of which is speckle effect. Speckle effect arises whenphase-coherent light, such as light from solid-state illuminators isscattered from a rough surface, such as a rough surface of a screen onwhich the images are displayed using the coherent light, and thescattered coherent light is detected by a detector having a finiteaperture, such as the viewer's eyes. An image displayed on the screenappears to comprise quantized areas with sizes around the size of thedetector's aperture. The intensities of the quantized areas in thedisplayed image often vary randomly, and such intensity variation (orfluctuation) is often referred to as the speckle effect.

In display applications using coherent light, such as light fromsolid-state illuminators, speckles accompanying the desired imagedisplayed on a screen overlap with the desired image, and thus maysignificantly degrade the quality of the displayed image.

Therefore, elimination or reduction of the speckle effect in displayapplications using phase-coherent light is highly desirable.

SUMMARY

In one example, a method for reducing speckle effect in a scanningdisplay system is disclosed herein, the method comprising: producing afirst phase-coherent light beam with a first polarization direction anda second phase-coherent light beam with a second polarization directionthat is different from the first polarization direction; and causing thefirst and second phase-coherent light beams to scan a display target soas to produce an image.

In another example, a scanning display system is provided, the displaysystem comprising: a light source capable of producing a first beam ofpolarized phase-coherent light; a depolarizer capable of producing fromthe first phase-coherent light beam second and third phase-coherentlight beams with first and second polarization directions; a firstscanner that is capable of causing the first and second phase-coherentlight beams to move along a first direction on a display target; and asecond scanner that is capable of causing the first and secondphase-coherent light beams to move along a second direction on a displaytarget

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a through FIG. 1 c schematically illustrate a typical scanningdisplay system that employs phase-coherent light; and

FIG. 2 a through FIG. 2 c schematically illustrate an exemplary scanningdisplay system with reduced speckle effect, wherein the display systememploys phase-coherent light.

DETAILED DESCRIPTION OF SELECTED EXAMPLES

Disclosed herein is a method of reducing speckle effect in scanningdisplay systems that employ phase-coherent light by driving light ofdifferent polarization directions from incoming polarized light; andusing the derived light for producing images in the scanning displaysystems. The speckle reduction method for use in scanning displaysystems will be discussed in the following, with particular examples,wherein the laser is used as phase-coherent light in scanning displaysystems. However, it will be appreciated by those skilled in the artthat the following discussion is for demonstration purpose, and shouldnot be interpreted as a limitation. Other variations within the scope ofthis disclosure are also applicable. For example, the method is alsoapplicable to scanning display systems that employ other types ofphase-coherent light.

Referring to the drawings, FIG. 1 a diagrammatically illustrates atypical laser-scanning display system (100) in the art. Laser from lasersource 102 is directed to X-scanner 104 that is capable of causing thelaser to move along the X direction, such as along the row of the imagepixels on screen 112. The laser from X-scanner 104 is directed toY-scanner that is capable of causing the laser to move along the Ydirection, such as along the column of the image pixels on screen 112.The laser (108) from Y-scanner 106 is directed to screen 112 to scan thescreen pixels for generating the desired images.

In monochromatic display application, the gray-levels of a displayedimage on the screen can be obtained by dynamically modulating theintensity of the light scanning the screen according to the desiredimage to be displayed (e.g. according to the bitplane of the desiredimage). In color display application, light of different colors (e.g.red, green, and blue colors) are provided; and the light of each coloris operated in the same way as that for the light of the single color.

The laser scanning on the image pixels of screen is better illustratedin FIG. 1 b. Referring to FIG. 1 b, screen 112 comprises an image pixelarray. The image pixel rows, such as rows 118 and 120 are along the Xdirection; and the image pixel columns are along the Y direction. In atypical scanning operation, the laser of the displays system generatesan illumination spot on screen 112; and the illumination spot has a sizearound that of an image pixel. For example as illustrated in FIG. 1 b,illumination spot 116 is caused by the laser. The illumination spot hasa size around the size of image pixel 114.

By moving the X-scanner and maintaining the position of the Y-scanner,the illumination spot (116) is caused to move along the X direction soas to sequentially scan through the image pixels in row 118. Afterscanning all image pixels in a row, such as row 118, the illuminationspot is moved to the next image pixel row, such as image pixel row 120by moving the Y-scanner; while the X-scanner can be moved to a locationsuch that the illumination spot (116) can be substantially aligned tothe first image pixel of the next row (e.g. image row 120). Afterscanning substantially all image pixels on the screen, the desired imagecan be produced.

Due to the phase coherency of the laser, the image displayed on thescreen (112) can be accompanied by a speckle pattern as diagrammaticallyillustrated in FIG. 1 c. For demonstration purposes, FIG. 1 cdiagrammatically illustrates an exemplary speckle pattern S₁ along a rowof image pixels, such as row 118 in FIG. 1 b, on the screen due tospeckle effect. The speckle pattern comprises speckles that appear to bequantized areas with randomly varying intensities to viewers. Specklesor quantized areas, such as quantized areas A and B, of differentintensities in the speckle pattern can be perceived by viewers.

The speckle effect in a scanning display system as discussed above canbe reduced by depolarizing the laser beam used for producing the desiredimages in the display system. Specifically, laser beams of differentpolarization directions are produce from a single laser beam (e.g. thelaser beam output from the laser source); and the produced laser beamsare used simultaneously to produce the desired image.

During a scanning process, the laser beams of different polarizationdirections generate separate illumination spots; and the illuminationspots are substantially aligned along the scanning direction, such asalong the rows of the image pixels on the screen. As the illuminationspots of laser beams with different polarization directions scan throughthe surface of the screen, each scanned diffusion point on the screensurface “sees” separate laser beams passing by with differentpolarization directions. Laser beams of different polarizationdirections interact differently with each diffusion point on the screen;and result in different and uncorrelated speckle patterns. The eyes ofan observer see decorrelated speckle patterns temporarily separated by atime interval equal to or less than the eye integration time. As aconsequence, the different speckle patterns are averaged out at the eyeretina resulting in the speckle reduction.

In one example wherein two laser beams with substantially orthogonalpolarization directions are produced from a single laser beam and theproduced two laser beams have substantially the same illuminationintensity, the speckle patterns perceived by viewer's eyes can bereduced by 3 db or 1/√2. Specifically, the contrast ratio between thebrightest area, corresponding to the area wherein the constructiveinterference occurs, and the darkest area, corresponding to the areawherein the destructive interference occurs, of the speckle patternsperceived by viewer's eyes (e.g. integrated by retina of the viewer'seyes) can be less than the contrast ratios of the individual specklepatterns. The speckle patterns appear to be less perceivable by viewers.

In order to maximize the speckle reduction, the difference between thepolarization directions of the produced laser beams is preferably 90°degrees. In other examples, the difference can be other values, such asa value larger than 0° degree and less than 90° degrees. The specklereduction is also maximized when the radiant flux carried by the twopolarized beams are substantially equal.

The depolarization can be accomplished by using a birefringent crystalwedge, as diagrammatically illustrated in FIG. 2 a. Referring to FIG. 2a, optical wedge 122 comprises an entrance facet (121 a) and an exitingfacet (121 b) that is not parallel to the entrance facet. A beam ofpolarized light (e.g. laser beam from a laser source) enters into theoptical wedge from the entrance facet (121 a); and can be split into twobeams 134 and 136. The two beams exit from the exiting facet (121 b)with different polarization directions. In one example wherein theoptical axis of the optical wedge-crystal is 45° degrees from thepolarization direction of incident light (124), the exiting light beams134 and 136 can have substantially orthogonal polarization directionswith substantially equal radiant flux.

The angle θ between the entrance facet (121 a) and the exiting facet(121 b) predominantly determines the angle between exiting light 134 and136. The angle between light 134 and 136 determines the distance betweenthe illumination spots generated by light 134 and 136 on the screen (asshown in FIG. 2 c, which will be detailed afterwards), which furtherdetermines the quality of the speckle reduction. It is preferred thatangle θ between the entrance facet (121 a) and the exiting facet (121 b)is equal to or less than 30° degrees, and more preferably between 5°degrees to 10° degrees and more preferably from 3° to 5° degrees so asto maximize the speckle reduction. The angle between the two exitingbeams is preferably 5 mrd or less, and more preferably 1 mrd. The idealangle between the two exiting beam is equal to the natural divergenceangle of each beam. In this way, the two polarized beams can becontacted to each other but without merging into each other. If theangle between the two beams is larger than the intrinsic beamdivergence, the resulting illumination spots on the screen might be toofar apart to be used as a single pixel. If the angle between the twobeams is smaller than the intrinsic divergence of the beams, the twoillumination spots on the screen might merge into each other and the twopolarizations won't have enough separation to generate the specklereduction. The typical divergence of a flying spot laser scanner can befrom 0.5 mrd to 5 mrd.

The optical wedge can be composed of a wide range of materials, such asLiNbO₃, quartz, YvO₄, Calcite, magnesium fluorides, mica, and many othersuitable materials. The optical wedge can be used with other opticalelements, such as an anti-reflection layer disposed on the entrancesurface. The optical wedge can also be combined with other opticalelements so as to form other suitable optical devices havingdepolarization functions, such as Wollaston prisms.

For demonstration purposes, FIG. 2 b diagrammatically illustrates anexemplary scanning display system that employs a depolarization opticalelement. In this example, the depolarization optical element is anoptical wedge as discussed above with reference to FIG. 2 a. Referringto FIG. 2 b, scanning display system 122 in this example comprises lasersource 102, optical wedge 126, lens 128, X-scanner 104, and Y-scanner106. Screen 112 may or may not be a member enclosed in the displaysystem.

Light source 102 provides polarized and phase-coherent light for thedisplay system. In one example, the light source is a solid-state lasersource, such as vertical cavity surface emitting lasers (VCSEL),extended cavity surface emitting lasers (e.g. NECSEL), and many othersuitable laser sources, capable of providing laser. Laser beam 124 fromlaser source 102 is passed through optical wedge 126 that splitsincoming laser 124 into two laser beams with substantially orthogonalpolarization directions. The laser beams exit from optical wedge 126 isdirected to X-scanner 104 through lens 128. X-scanner 104 in thisexample comprises a reflective surface by which laser beams from lens128 are directed towards Y-scanner 106. Movement of the X-scanner can beaccomplished by attaching the reflective surface to a moving mechanism,such as a step motor.

The laser beams (e.g. 130 and 132) reflected from X-scanner 104 arereflected by Y-scanner 106 towards the screen (112) so as to generatethe desired image. As discussed above, the gray-levels of image pixelscan be achieved by dynamically adjusting the intensity of the light.Colors of image pixels can be achieved by scanning the screen usinglight beams of different colors, such as red, green, and blue colors.

In a scanning process, the produced laser beams (e.g. 130 and 132) eachgenerate an illumination spot at the screen; and the illumination spotsare substantially aligned along the row of the image pixels on thescreen as diagrammatically illustrated in FIG. 2 c. Referring to FIG. 2c, illumination spots 138 and 142 are generated by laser beams 130 and132 respectively. The distance L between the centers of the illuminationspots 138 and 142 can be any suitable value. In one example, thedistance L can be around the size D of an image pixel on the screen, orcan be around the size of the pitch P of image pixel array on thescreen, wherein the pitch is defined as the distance between theadjacent image pixels in a row of the image pixel array on the screen.For example, the distance L can be from 0.1 mm to 10 mm, such as from 1mm to 5 mm. Each illumination spot may have a characteristic dimensionaround or less than the size of an image pixel on the screen. Thedistance L can be as small as possible without having the beams merginginto each other. Because the two beams are derived from one single laserbeam and the intensity modulation (for achieving the gray-levels ofimage pixels) is done by changing the power supply intensity on thelaser. It is therefore preferred (though not required) that theillumination spots generated by the two laser beams on the screen are asclose as possible in order to remain within one single image pixel.

The size of each illumination spot and/or the distance between thecenters of the illumination spots can be controlled by optical lens 128.Specifically, by adjusting one or both of the focal length and therelative position of lens 128 between optical wedge 126 and X-scanner104, sizes of the illumination spots and/or the distance between thecenters of the illumination spots 138 and 142 (in FIG. 2 c) can beadjusted for optimal speckle reduction.

The illumination spots (138 and 142 as illustrated in FIG. 2 c) arecaused to move along the row of the image pixels (e.g. along the Xdirection) by moving the X-scanner 104 (as illustrated in FIG. 2 b)while maintaining the position of the Y-scanner 106. Because the laserbeams (130 and 132 as illustrated in FIG. 2 c) generating theillumination spots 138 and 142 have different polarization directions,such as orthogonal polarization directions, each diffusion point on thescreen causes separate and uncorrelated speckle patterns as theillumination spots pass by. When the viewer views the image displayed onthe screen, the viewer's eyes integrate the separate speckle patterns;and the integrated speckle patterns in the viewer's eyes appear to haveless contrast as compared to individual speckle patterns. As aconsequence, the integrated speckle patterns in the viewer's eyes appearless perceivable than either one of the individual speckle patterns.

After scanning across the entire row of the image pixels on the screen,the illumination spots are moved to the next row for scanning the imagepixels on the next row. This movement can be accomplished by moving theY-scanner for aligning the illumination spots to the next row of imagepixels on the screen; and moving the X-scanner for aligning one of thetwo illumination spots to the starting image pixel (e.g. the left-mostimage pixel) of the next row. The above scanning process continues untilsubstantially all image pixels on the screen are scanned.

It is noted that the scanning display system as discussed above withreference to FIG. 2 b is only one of many possible scanning displaysystems in which examples of the speckle reduction method of thisdisclosure can be implemented. Scanning display systems with otherconfigurations are also applicable. For example, the X-scanner (104) andthe Y-scanner (106) in FIG. 2 b can be exchanged. Specifically, theY-scanner (106) can be disposed between the X-scanner (104) and lightsource 102 on the optical path of the scanning display system. Thescanning display system may comprise other suitable optical elements,such as a field lens, a relay lens, and a projection lens.

It will be appreciated by those of skill in the art that a new anduseful method for speckle reduction and an optical system capable ofspeckle reduction have been described herein. In view of the manypossible embodiments, however, it should be recognized that theembodiments described herein with respect to the drawing figures aremeant to be illustrative only and should not be taken as limiting thescope of what is claimed. Those of skill in the art will recognize thatthe illustrated embodiments can be modified in arrangement and detail.Therefore, the devices and methods as described herein contemplate allsuch embodiments as may come within the scope of the following claimsand equivalents thereof.

1. A method for reducing speckle effect in a scanning display system,the method comprising: producing a first phase-coherent light beam witha first polarization direction and a second phase-coherent light beamwith a second polarization direction that is different from the firstpolarization direction; and causing the first and second phase-coherentlight beams to scan a display target so as to produce an image.
 2. Themethod of claim 1, wherein the step of producing the first and secondphase-coherent light beams comprises: producing the first and secondlight beams from a beam of polarized phase-coherent light.
 3. The methodof claim 2, wherein the step of producing the first and secondphase-coherent light beams further comprises: producing a thirdphase-coherent light; and producing the first and second phase-coherentlight beams by passing the third phase-coherent light beam through abirefringent crystal wedge.
 4. The method of claim 2, furthercomprising: directing the first and second light beams towards a firstscanner that is capable of causing the first and second light beams tomove along a first direction on the screen; and directing the first andsecond light beams towards a second scanner that is capable of causingthe first and second light beams to move along a second direction on thescreen.
 5. The method of claim 4, wherein the first direction is along arow or a column of an array of image pixels on the screen; and thesecond direction is along a column or a row of the array of image pixelson the screen.
 6. The method of claim 2, wherein the first and secondphase-coherent light beams are laser beams.
 7. The method of claim 2,wherein first and second light beams has an angle therebetween; andwherein the angle is 5 mrd or less.
 8. The method of claim 2, whereinthe first beam generates a first illumination spot on the screen; andthe second beam generates a second illumination spot on the screen; andwherein the first and the second illumination spots are separated by adistance that is equal to or less than a size of an image pixel on thescreen.
 9. The method of claim 2, wherein the first beam generates afirst illumination spot on the screen; and the second beam generates asecond illumination spot on the screen; and wherein the first and thesecond illumination spots are substantially aligned along a row of anarray of image pixels on the screen.
 10. The method of claim 3, whereinthe wedge comprises an entrance facet from which an incident beam entersinto the wedge; and an exit facet, from which the first and second beamsexit from the wedge; and wherein the entrance and the exit facets havean angle that is from 3° degrees to 5° degrees.
 11. The method of claim1, further comprising adjusting an intensity of the first and the secondphase-coherent light according to a gray-level of an image pixel.
 12. Ascanning display system, comprising: a light source capable of producinga first beam of polarized phase-coherent light; a depolarizer capable ofproducing from the first phase-coherent light beam second and thirdphase-coherent light beams with first and second polarizationdirections; a first scanner that is capable of causing the first andsecond phase-coherent light beams to move along a first direction on adisplay target; and a second scanner that is capable of causing thefirst and second phase-coherent light beams to move along a seconddirection on a display target.
 13. The system of claim 12, wherein thedepolarizer is a birefringent wedge.
 14. The system of claim 13, whereinthe wedge has an optical axis that is substantially 45° degrees from thepolarization direction of the first phase-coherent light beam.
 15. Thesystem of claim 13, wherein the wedge has an entrance facet and an exitfacet, wherein the exit facet has an angle with the entrance facet; andthe angle is from 5° to 10° degrees.
 16. The system of claim 12, whereinthe first and second polarization directions is substantiallyorthogonal.
 17. The system of claim 15, wherein the first and secondphase-coherent beams have an angle therebetween; and said angle is 5 mrdor less.
 18. The system of claim 15, comprising: an optical lensdisposed between the depolarizer and one of the first and secondscanners on an optical path of the display system such that each one ofthe first and second phase-coherent beams creates an illumination spoton the screen; and the illumination spot has a dimension that issubstantially equal to or less than a size of an image pixel on thescreen.
 19. The system of claim 15, comprising: an optical lens disposedbetween the depolarizer and one of the first and second scanners on anoptical path of the display system such that each one of the first andsecond phase-coherent beams creates an illumination spot on the screen;and a center-to-center distance between the illumination spots is equalto or less than a size of an image pixel on the screen.
 20. The systemof claim 12, wherein the first or the second beam is a laser beam.
 21. Amethod for reducing speckle effect in a laser scanning display system,the method comprising: providing a first laser beam; deriving second andthird laser beams from said first laser beam such that the second andthe third laser beams have different polarization directions; andscanning a display target by the second and the third laser beams so asto generate an image on the display target which has a perceived speckleeffect less than a perceived speckle effect producible by the firstlaser beam alone.
 22. The method of claim 21, wherein the step ofderiving the second and the third laser beams comprises: passing thefirst laser beam through a birefringent crystal wedge.
 23. The method ofclaim 22, wherein the second laser beam has a polarization directionthat is substantially perpendicular to the polarization direction of thethird laser beam.
 24. The method of claim 23, wherein the step ofscanning the display target comprises: generating an illumination spoton the display target by the second laser beam; and generating anotherillumination spot on the display target by the third laser beam, whereina center-to-center distance between the two illumination spots issubstantially equal to a size of an image pixel on the display target.25. The method of claim 21, comprising: changing an intensity of saidfirst laser beam according to an gray-level of an image in an image tobe displayed.